The present invention relates to steam generators, particularly waste-heat boilers, and in particular the present invention relates to apparatus for supplying feedwater to such a steam generator.
The present invention relates in particular to apparatus for feeding to such a steam generator feedwater which is in a degassed condition, the feedwater being heated to the boiling temperature in order to be placed in its degassed condition.
It is already known, in order to avoid corrosion from water in a steam boiler, to degas the treated boiler feedwater in a degasser at boiling temperature, with hydrazine often being utilized for any residual degasification. In the case of fossil-fired steam boilers, the supply of feedwater takes place at temperatures above 100.degree. C, so that the degassing of the feedwater can take place in the preheating circuit of the feedwater. However, in the case of steam boilers, particularly waste-heat boilers, where there is no appreciable preheating of feedwater by exhaust steam of a turbine, the degassing of the feedwater is provided by directing part of the generated steam, through a suitable branch conduit, to a feedwater degassing means, approximately 15% of the generated steam being utilized for this purpose. The degasser will then deliver feedwater at the boiling temperature, so that the feedwater temperature is predetermined.
A method and apparatus of this latter type is thermodynamically uneconomical, inasmuch as an undesirably large portion of the generated steam at a relatively high temperature must be cooled to a relatively low temperature level in order to heat the feedwater to its boiling temperature. Moreover, a method and apparatus of this type sets a high limit on the discharge temperature of the boiler.